Mitochondrial complex I activity is impaired in Parkinson's disease (PD), and inhibition of complex I with MPTP or rotenone reproduces many features of PD in animal models. The complex I defect can be transferred to cell lines expressing mitochondrial DNA (mtDNA) from PD patients, suggesting that mtDNA mutations account for the complex I defect. But despite attempts to identify them, the specific mutations that account for this defect remain unknown. Mitochondrial complex I dysfunction increases free radical production in the mitochondria, resulting in damage to macromolecules, with particularly high levels of potentially mutagenic damage to mtDNA. This damage to mtDNA accumulates with age and reaches especially high levels in PD. We hypothesize that this oxidative damage to mtDNA leads to the accumulation of somatic mtDNA mutations, ultimately contributing to the loss of dopaminergic terminals and potentially to cell death. Therefore, we predict that substantia nigra (SN) neurons will harbor high levels of somatic mtDNA mutations at early stages of PD. Consistent with this prediction, we present preliminary data indicating remarkably high levels of somatic mtDNA point mutations in SN neurons at very early stages of PD, whereas neurons with high levels of mutations are largely absent by end stage PD. Furthermore, we find that levels of the subset of mtDNA mutations predicted to result from oxidative stress are nearly 10-fold more prevalent in SN neurons from early PD compared to controls or to late PD neurons. These data are consistent with our hypothesis that somatic mtDNA mutations accumulate in SN neurons at early stages of PD, and that these mutations contribute to neuronal loss in PD. We further predict that experimental acceleration of the age-related accumulation of somatic mtDNA mutations will lead to similar changes in transgenic mice expressing a proofreading deficient mtDNA polymerase (POLG). We propose to use laser capture microdissection to analyze point mutations and large deletions in neurons and glia from human postmortem SN neurons and other brain regions in early PD, late PD, and controls. We further propose to conduct parallel experiments in transgenic mice expressing mutant POLG. Together, these studies have the potential to reveal a key mechanism in the pathogenesis of PD, and may lead to novel neuroprotective strategies.PUBLIC HEALTH RELEVANCE: Parkinson's disease (PD) is a common disorder that leads to progressive disability. Though many symptomatic treatments exist for PD, each has limitations, and a strategy to slow the progression of PD could have an enormous positive impact on the quality of life of PD patients. The proposed experiments will test the hypothesis that the accumulation of somatic mitochondrial DNA mutations in the brain contributes to the pathogenesis of PD, and may lead to novel strategies to slow the progression of PD.